Thin walled natural rubber and synthetic polyisoprene film products intended for medical uses or other contact with human tissue are advantageously made of vulcanizates with a certain combination of properties. In most cases, it is desirable to combine a relatively low 100%, 300% or 500% tensile modulus with very high ultimate tensile strength and ultimate elongation values, and with high tear strength properties. Rubber products that may benefit from such enhanced tensile properties include, for example, medical gloves, condoms, male external urinary drainage catheters, surgical tubing, contraceptive diaphragms, finger cots, catheter balloons and cuffs, uterine thermal ablation balloons, drug infusion bladders, tissue retrieval pouches, medical tubing, baby bottle nipples, infant pacifiers, anesthesia breather bags, resuscitation bags, rubber dental dams, exercise bands and surgical tubing.
With respect to surgical gloves, it is important to have low 100% tensile modulus values, combined with high tensile strength. This is a difficult combination to achieve, as the lower modulus usually produces a lower tensile strength material. A relatively low tensile modulus is necessary to ensure that such gloves remain comfortable during use. If the tensile modulus is too high, the user's hands may become fatigued over time as progressively more strength is required to stretch the glove material. This is particularly problematic with gloves that are to be used for a prolonged period of time such as for a long surgical procedure. A combination of low modulus with high tensile strength is necessary to provide such desired comfort along with a very large safety margin with respect to glove failure.
Low tensile modulus values are also important in condoms to promote ease of donning, and in catheter balloons where ease of inflation is beneficial. A low tensile modulus is also of value in elastomeric drug infusion bladders, making it easier to fill the bladder with a drug solution.
Another tensile property affecting the usefulness of certain thin walled medical and personal film products is tear strength, important for preventing premature failure. Baby bottle nipples and baby pacifiers benefit from high tear strength since this prevents the child's teeth from severing the nipple or pacifier during use
For catheter balloons, it is very important to combine high tear strength with high elongation to protect the balloon from bursting during use. For condoms, the combination of exceptionally high tear strength and tensile strength combined with high ultimate elongation is very desirable. For exercise bands, it is desirable to have very high ultimate elongation, combined with high tensile strength. Because exercise bands are often packed for travel and can impart unpleasant odors to packed clothing, it is very advantageous to have a band with very low odor.
With respect to rubber dental dams, it is very important to combine very high elongation with very high tensile strength to allow for the placement of the rubber dam over and around a tooth without risk of failure by tearing. Also it is very desirable for dental dams to have low levels of odor and taste.
Many rubber products must meet certain standards in order to be marketed. For example, medical devices, such as surgical gloves, examination gloves and condoms must meet the tensile strength, ultimate elongation and tactility standards of the American Society for Testing and Materials (ASTM). The ASTM has established Standard D3577-88 for rubber surgical gloves, D3578-77 for examination gloves, and D3492-83 for condoms. Each of these standards establishes a minimum ultimate tensile strength for the specified product under particular conditions. The minimum ultimate tensile strength specified for Type I (natural latex) surgical gloves in ASTM D3577-88 is 24 MPa. The minimum ultimate tensile strength specified for Type II (Synthetic rubber latex or rubber cement) surgical gloves in ASTM D3577-88 is 17 MPA. For condoms in ASTM D3492-83, a minimum of 17 MPA is required. The minimum ultimate tensile strength specified for natural rubber examination gloves in ASTM D3578-00 is 14 MPA. The ASTM standards also establish maximum deformation stress at 500 percent elongation.
Natural and synthetic rubber have been used extensively as materials for thin walled medical and personal film products. The highest durability and flexibility are provided by a rubber film that is seamless and of uniform thickness. This is best achieved when the thin walled rubber products are made by dip-molding and/or casting of the film. Dip-molding and/or casting of rubber is performed with either a latex (an aqueous dispersion of rubber particles) or an organic solution of the rubber. Dip-molding in either the latex or the organic solution is followed by removal of the water or solvent; the dipping and water or solvent removal are often performed in repeated cycles to achieve a particular film thickness. The film thus formed is then vulcanized to bring the rubber to a fully cured state. Latex can be processed without breaking down the molecular weight of the rubber, whereas dry-rubber methods, which utilize high shear to comminute the rubber and combine it with other compounding ingredients for processing, tend to degrade the molecular weight.
It is generally known that rubbers that are crosslinked only through carbon-carbon bonds have inferior tear strengths as compared with rubbers that contain sulfidic and/or polysulfidic crosslinks. Vulcanization, particularly with sulfur, has traditionally been performed in the presence of vulcanization accelerators. The most widely used accelerators are those that contain secondary amino groups, such as dialkylamino groups, cycloalkylamino groups, and morpholinyl groups. Secondary amino groups are found, for example, among the traditional sulfenamide, dithiocarbamate and thiuram accelerators. An unfortunate consequence of the inclusion of these accelerators is their tendency to produce an adverse reaction in individuals with whom the resulting rubber articles may come into contact. The reaction is commonly referred to as a Type IV allergy, which is mediated by T cells, generally occurs within six to 48 hours of contact with the rubber article, and is localized in the area of the skin where contact is made. Secondary amine-containing accelerators are also referred to as nitrosatable amines since they are susceptible of reaction with atmospheric nitrogen oxides during mixing, milling, extrusion, molding, calendaring, curing, and even warehousing and storage, to produce nitrosamines, which have been identified as potential human carcinogens.
Typically, sulfur vulcanization in the absence of an accelerator leads to rubber products with undesirable tensile properties. It would be advantageous to perform accelerator free vulcanization while achieving optimal strength of the rubber product thus produced.
Because of the disadvantages of the sulfur vulcanization process, it has become important to develop crosslinking methods that provide useful tensile properties with minimal toxicological properties. Crosslinking agents that provide rubber vulcanizates 1) free of reaction byproducts, 2) without the need for accelerating agents, and 3) through a low temperature curing protocol, while maintaining practical physical properties would be invaluable. Polynitrile oxides (PNOs) react readily with unsaturated molecules because they participate in a 1,3-dipolar cycloaddition with a variety of multiple bond functional groups. In the reaction of a PNO with ethylenic points of unsaturation, the cycloaddition product is an isoxazoline ring. Reaction of a rubber compound with a PNO therefore provides crosslinking regions within the polymer comprised of two or more isoxazoline units, usually separated by an aromatic structure.
The high reactivity of PNOs allows for crosslinking to occur at lower temperatures than with other non-accelerated vulcanization reagents and without the formation of byproducts (i.e., virtually all atoms of the reactants are incorporated into the rubber structure). These advantages of PNO reactivity has cultivated interest in their use as crosslinking agents for a variety of elastomeric polymers.
For example, the use of polynitrile oxides (PNOs) as low temperature crosslinking agents for various types of unsaturated rubber and other polymeric materials is known. Breslow, et al. in U.S. Pat. No. 3,390,204 disclose the use of various polynitrile oxides to crosslink unsaturated polymers. Possible articles of manufacture from such vulcanizates are also listed, and include items such as tires for motor vehicles, tubing, and pipes. Breslow specifically states that the cross-linked polymers are hard, tough resins. The only physical property disclosed by Breslow is the higher tensile strength of the vulcanizates as compared with the non-vulcanized starting rubber.
Lysenko, et al. in WO 97/03034 disclose the use of a dispersion of stable polynitrile oxides useful in latex materials. Specifically, the use of 2,4,6-triethylbenzene-1,3-dintrile oxide (TON-2) is cited as a useful one part room temperature crosslinking agent for latices. Lysenko also notes the utility of TON-2 for crosslinking various polymers to create useful one-part coatings. There is no mention of TON-2 or other polynitrile oxides imparting any special physical properties to articles of manufacture. No physical properties of articles or coatings made with TON-2 are disclosed.
Stollmaier, et al. in U.S. Pat. No. 6,753,355 references the utility of TON-2 for crosslinking various latex polymers for producing foam rubber articles, including flooring, wall covering, shoe lining, and non-woven materials. Polyisoprene is listed as one of a number of potential latices from which foam rubber backings can be made with the use of TON-2. Only foam containing products are disclosed as articles of manufacture.
Parker in U.S. Pat. No. 6,355,826 discloses an improved method of synthesizing mesitylene dinitrile oxide (MDNO). Parker cites the use of MDNO and other polynitrile oxides in the coating of fabrics with rubber-based coatings. Parker states that stable nitrile oxides are desirable from the perspective of handling, as compared to unstable polynitrile oxides. The Parker patent contains an extensive list of references to prior uses of MDNO and other polynitrile oxides, which are incorporated herein by reference.
Breton, et al. in U.S. Pat. No. 6,252,009 discloses the use of polynitrile oxides for making highly solvent resistant thermoplastic vulcanizates. V. V Boiko and I. V. Grinev, “Influence of MDNO/Processing Elastomers,” International Polymer Science and Technology, vol. 22, No. 7, T/21, 1995 cite the utility of MDNO for increasing the Mooney viscosity of a synthetic polyisoprene rubber during solid rubber milling operations.
M. G. Vlasyuk, et al. in “Chemical and toxicological health studies of elastomer compositions containing dinitrile oxide” International Polymer Science and Technology, 23, No. 7, 1996 discloses the use of fabric coated with a solvent solution of synthetic polyisoprene vulcanized with MDNO. The toxicological properties of the coated fabric are revealed. No physical properties of the coating itself or of the coated fabric are provided. Latex is not used. Unsupported films are not disclosed.
Russian Patent SU 2,042,664 discloses the physical properties of various polymers cured with bis-nitrile oxide. Attention is given to those polymers of very low unsaturation, including butyl rubber, urethane rubber, polysiloxane rubber and ethylene propylene rubber. These polymers are generally difficult to crosslink, and MDNO is thought to provide an advantage to the crosslinking of these polymers, in that MDNO allows for crosslinking with modest time and temperature conditions, including room temperature conditions. Table 1 of this patent shows that tensile strength values were only slightly better for the MDNO cured polymers than for prior art curing systems. No physical property data was provided for polyisoprene or natural rubber, as they are highly unsaturated polymers.
McGlothlin, et al. in U.S. Patent Application 2004/0071909 disclose high performance rubber vulcanizates from latex for producing thin walled rubber articles. Such vulcanizates contain a combination of carbon-carbon bonds, mono and polysulfidic crosslinks, without the use of components that contain secondary amine groups or any nitrosatable substances which have a tendency to convert to nitrosamines under certain conditions. The disclosed vulcanization method offers significant advantages over prior art, especially as compared to organic peroxide vulcanized rubber articles of manufacture. However, further improvements in physical properties are still desirable.
McGlothlin et al. in U.S. Pat. No. 6,329,444 disclose the use of sulfur-free, free-radical-cured cis-1,4-polyisoprene for use in dip-molded medical devices. Vulcanizates made by this method are free of undesirable accelerators and can have very low odor and be non-cytotoxic. However, physical properties of the vulcanizes are generally lower than for prior art accelerated sulfur vulcanizates.
McGlothlin, et al. in U.S. Pat. No. 6,775,848 disclose a method of secondary vulcanization by imbibing additional vulcanizing agent into an already partially vulcanized article. No disclosure is made to the possibility of incorporating a vulcanizing agent into a film for primary vulcanization.
Miller, et al. in U.S. Pat. No. 5,039,750 discloses the addition of small amounts of styrene butadiene latex added to traditional sulfur accelerated natural rubber latex in an attempt to improve tear strength and tensile strength properties. Modest improvements in both properties were noted. No improvement to odor would be expected, nor is there any note of the utility to latices which are free of sulfur and vulcanization accelerators.
Amnad in U.S. Pat. No. 5,872,173 discloses the use of silica added to synthetic latex to improve tear strength.
Amdur et al in U.S. Pat. No. 5,458,588 cites the use of dispersed silica in the prior art sulfur accelerated vulcanizing of natural rubber latex to improve the tensile strength, tear strength, wet strength, break force, puncture and tear resistance. While the silica addition appears to modestly improve both the tensile strength and the tear strength, it does so to the detriment of increasing the tensile modulus at low elongations, and to the detriment of ultimate elongation.
Evans, et. al, “Microencapsulated Antidegradants for Extending Rubber Lifetime” Rubber Chemistry and Technology Volume 65 No. 1, pp. 201-210 discloses the use of microencapsulation technology for extending the life of rubber compounding agents within compounded dry rubber. Evans also teaches state of the art technologies involved in microencapsulating chemical agents for use in rubber compounds. There is no disclosure in this publication of extending the life of vulcanization agents within compounded latex.
To the knowledge of the present inventors, there are no prior art publications which disclose the use of polynitrile oxides as suitable vulcanization agents for thin walled rubber film products made by dip-molding or casting and intended for direct or indirect contact with living tissue.
Indeed, polynitrile oxides are exceptionally reactive materials, especially with respect to compounds that contain multiple bonds, including the double bonds of rubber materials. This rapid rate of reaction can result in desirable crosslinking when the polynitrile oxides are used as vulcanizing agents; however it may also lead to extensive pre-vulcanization (i.e., rubber crosslinking that occurs prior to the dipping or casting stage). Although several prior art references address the use of polynitrile oxides as crosslinking agents for natural rubber and synthetic cis-1,4-polyisoprene rubber, none of these references teach a method to retard or prevent pre-vulcanization. As far as presently advised, no mention of pre-vulcanization of PNO crosslinked natural rubber or polyisoprene film products is made in the prior art; however, the methods employed suggest that it would be extensive.
As discussed in SU 2,042,664, cited above, the use of PNOs may be very desirable for crosslinking rubber materials that have very low levels of unsaturation. In that case, the high reactivity of the polynitrile oxide compensates for what would otherwise be a very slow vulcanization process (i.e., with traditional sulfur accelerated cure packages). However, when used to crosslink highly unsaturated materials such as natural rubber and synthetic polyisoprene, the high rate of reaction of PNOs makes them more reactive than even the fastest of the “ultra accelerators” used in sulfur cure systems. At first glance, it would appear that polynitrile oxides would be unacceptable crosslinking agents for thin walled rubber film products because complete pre-vulcanization crosslinking would interfere with the dip-molding or casting process.
Pre-vulcanization of solid rubber materials is a well-known problem. During compounding of dry rubber in rubber mills and the like, heat is generated, which can cause some vulcanization to begin. In some cases, this is desirable. In other cases, it is not. For instance, some vulcanization at this stage might improve the green strength of low molecular weight rubber materials. During the molding or extruding of rubber compounds, suitable accelerators are chosen to avoid too much unwanted early vulcanization of the material (“scorch”) during processing. Rubber compounders select vulcanization accelerators, which do prevent the scorch from interfering with the curing process. With latex processes, it is permissible to use very rapid accelerators, such as dithiocarbamates, which would not be suitable for dry rubber. Because latex is processed at relatively low temperature, even the so-called “ultra accelerators” can be used, as even these are not too active in relatively cold latex compounds. For instance, it may take a matter of days before a latex compound is ruined due to too much unwanted pre-vulcanization at room temperature.
However, in the case of polynitrile oxide crosslinked rubber film products, if nothing is done to restrict pre-vulcanization, the resulting tensile strength properties of the product films can be about 50% of what they otherwise would have been. The reason for this is that traditional latex compounding practice for making such dip-molded or cast films is virtually always followed by a “maturation” or resting period. This maturation period is generally long enough for most of the bubbles to come out of the newly compounded latex but can also allow for extensive pre-vulcanization if methods are not employed for its prevention. Thus, in the normal course of screening polynitrile oxides as candidate crosslinking agents for rubber film products, the polynitrile oxides may well have been eliminated, in the past, from further consideration due to the poor physical properties resulting from the extensive pre-vulcanization during maturation.
Accordingly, it is among the objects of the present invention to provide thin walled dip-molded or cast rubber film products crosslinked by polynitrile oxide crosslinking agents, which have been prepared without substantial pre-vulcanization and which have improved tear strength, ultimate elongation and other properties.
A further object of this invention is to provide a method for forming such products.
Other objects and advantages of the products and methods of the invention will be apparent from the following description of preferred embodiments thereof.